Despite repeated appeals by the health and environmental authorities, the volume of municipal solid wastes—and of the waste which may be assimilated thereto—keeps growing, both because the well-being currently enjoyed in a part of the world entails generally growing consumptions, and because the very health authorities demand bulkier packages and often the disposable use of objects which have stringent hygienic requirements. Waste treatment is thereby a general problem and is particularly serious in the most highly industrialised countries, where space is at a premium.
So far, also due to issues of acceptance by the population towards the other disposal methods, waste has been mostly dumped in controlled landfill sites. Such system turns out to be increasingly inadequate, since it requires a continuous supply of space, preferably away from residential areas and entails various forms of environmental pollution; in particular, the liquids coming from waste decomposition and containing various toxic substances often leach into the underground aquifer.
The incineration of municipal solid waste, a practice which was once a source of various toxic gases, is now carried out with greater care, so as to produce virtually only carbon dioxide fumes and water, which over time originate also in a landfill site. The improvement of these processes, as well as their use for heat and/or electric energy generation, is a reality in the most advanced countries and will spread ever more.
At the end of the incineration process, between 20% and 30% of the fed waste remains in the form of incinerator bottom ash and/or debris (also known as IBA or bottom slag), essentially consisting of the non-combustible fractions and made up mainly of vitreous and/or ceramic materials, coming from the chemical-physical transformations of the combustion process, especially for the high temperatures (about 900° C.) reached in a correctly managed incinerator.
In Italy and in Europe, bottom ash and/or debris from the incineration of municipal solid waste and assimilated waste as well as from RDF (Refuse-Derived Fuel), are classified as non-hazardous waste by the so-called CER 190112 code. It may be that, according to domestic rules or specific authorisations, they are possibly mixed, also in the present invention, with other waste having the same origin.
Such bottom ash and/or debris are normally disposed of in a landfill site. Some suggestions have also been put forward to reuse the bottom ash and/or debris, the amount of which is definitely remarkable, with particular reference to the use thereof as material for the building industry.
Some studies show how bottom ash and/or debris, consisting mainly—as seen—of vitreous and ceramic components, have a pozzolanic potential. However, said bottom ash and/or debris, in addition to being granulometrically coarse and being rich in metals and various impurities, also contain non-negligible amounts of amphoteric metals, such as particularly aluminium and zinc which, in an alkaline environment and hence even more so in water-concrete mixtures, give rise to complex ions, with generation of gaseous hydrogen which produces expansion and, eventually, spoils completely the concrete product.
The same Applicant, in application PCT WO02/081 398, had suggested the wet milling and the aqueous suspension of bottom ash and/or debris to use them as additives for concrete and/or for concrete conglomerates. A use of this type is certainly interesting, but leads to the manufacturing of products of a rather limited added value.
EP 1 382 584 explained that, during the wet shredding of bottom ash and/or debris, corrosion by oxidation of the aluminium therein contained takes place and provided the preliminary shredding of bottom ash in aqueous suspension and the subsequent use for the manufacture of concrete. Later works, in the frame of university degree thesis, carried out or directed by the same authors of EP 1 382 584, whereto also the inventor of the present intention took part, highlighted how the simple wet shredding was often not sufficient to solve the problem of hydrogen building and they considered it suitable to increase pH by adding of all-but-negligible amounts of concrete (10-15% of the dry fraction), furthermore explicitly excluding resorting to other alkaline agents which might have determined in the concrete the well-known and deleterious reaction “alkali/aggregates”. The addition of cement, on the contrary, was not a solution, because, in addition to implying significant costs items, while on the one hand it accelerated metal corrosion, on the other hand it also caused a phenomenon of cement curing and of reaction of the same with the shredded ash, so that firstly aqueous suspensions became even more difficult to be treated industrially, secondly, sort of lumps were generated, consisting of cement and ash, having very low mechanical resistance, due to the high amount of malm water. These friable cement/ash lumps were disaggregated by the lab cement mixers used in experiments, by which the mixing energy lied far above that, for example, of concrete-mixing equipment, where cement mixers are generally no longer used. These additions of cement, hence would have transferred to concrete manufacturers—the end users of these aqueous suspensions—two problems: even more unstable suspensions and cement/ash lumps to be disaggregated during the mixing phase.
Therefore, both the solutions set out above for reusing the bottom ash and/or debris provided the manufacture of a water suspension to be sold or used as such on the market of cement products, essentially for the manufacture of concrete. However, such a product:
it was not certifiable which addition of mineral ore of a pozzolanic nature, according to current standards on the subject of concrete (concrete, among other things, as known, being a material used for structural purposes, must undergo long and complex procedures to implement innovations);
it should have been sold or used by a very high number of concrete manufacturers and/or prefabs and/or concrete manufactured items (realistically over 20 for a medium-sized incinerator) and this would have been made even more critical by the fact that the manufacturing plants of these subjects are not equipped for receiving, in large amounts, water suspensions, but commonly receive products in the form of powder; moreover in the form of water suspension the products is partly unstable, since it is subject to decanting and sedimentation and this determines the need to maintain it in motion before use;
it presented a variability of the features, possibly linked to seasonal events, such as the variation of the features of the waste during the Christmas period, which would have affected downstream too large a number of companies, which would have hardly been willing to re-calibrate the mixture of concrete due to a very minor component;
it would have encountered difficulties in obtaining the authorisation for use, since it would have been—at least for a not exactly short period of time—a unicum on the market and the recovery process would have been completed by various third-party users making the entire recovery cycle and the degree of potential pollution in fact less traceable from an environmental point of view.
Moreover, these inventions and studies did not pick two absolutely essential elements to solve in a short time and industrially sustainable the problem of corrosion of amphoteric metals: the features of granulometric fineness which the product was to have and the opportunity to use alkali to accelerate the corrosion process of aluminium. As a matter of fact, they did not highlight the fact that the wet shredding was supposed to allow to reduce the size of all the particles below a certain threshold size and that, on the contrary, the average size or even the size below which 90% of the particles lay was not particularly important. As a matter of fact, the metallic aluminium found in bottom ash being very ductile, it is among the most difficult fractions to be shredded, so it is pointless to grind the ash to an average fineness of about 3 μm if then no attention is given to the fact that the granulometric curve has a 3% fraction lying in the range between 50 μm and 60 μm: the maximum size of the particles were and are essential.
It is highlighted how the granulometric characteristics reported both in WO02/081 398 and in EP 1 382 584 consider—more or less explicitly—as fundamental the achievement of a high granulometric fineness, however without picking the most important aspect i.e. the one concerning the maximum size of the particles. For example it is apparent how in a drawing reported in EP 1 382 584, while the average size of the wet-shredded particles are about 3 μm (high degree of fineness), there is about 3% of particles having a size ranging between 50 and 60 μm (larger size than that concerning dry shredding, which sets D50 about 4.5 μm and D98-D100 about 28-30 μm): the corrosion of these particles will certainly be far slower. This is extremely important because, if the main purpose of grinding is that of achieving a high average value, it is incorrect. Also WO02/081 398 pays no attention to the fineness in the sense of the maximum size of the particles, so much so that it even indicates only the values below which must be 90% of the particles. Due to the above, during the grinding and in those activities directly connected to the same, it is useful to set in place devices, to achieve the purpose of causing all the particles to be below a certain size.
Italian patent application no. MI93A 002650 shows the use of a ternary mixture of ash, calcium sulphate and lime for obtaining a manufactured item for the building industry having high mechanical resistance. The patent refers specifically to the ash which is left from the manufacture of coal and is particularly directed at fly ash (the lightest one). As regards the opportunity to use bottom ash (again generated in the combustion of coal), it is stated the opportunity that the same be used, but substantially as “aggregates” and not as a binding component. Moreover, in order to achieve these results, the starting point is a relatively even supply, such as the combustible coal of power plants, while nothing is said on completely different compositions, which are more variable and more complex, such as the charges of incenerators of solid municipal waste. It is hardly appropriate to highlight how various types of thermal processes exist and how the residues of each of these thermal processes depends on the chemical-physical characteristics of the incoming material, on the temperatures, on the combustion times, on the calorific value of the fuel, etc.